Our general objective is to study the development and replication of cellular organelles using Euglena, chiefly, as a convenient research organism. Our present objective is to understand the induction and regulation of organelle development by light and other substrates. The major question to be answered ultimately is how light regulates the biosynthesis of plastid and mitochondrial constituents by controlling the availability of energy, molecular constituents, and genetic information from the plastid, the mitochondrion and the rest of the cell. At this point in our work our primary interest is in the control of the biosynthesis of plastid thylakoid membrane constituents including protochlorophyll(ide) pigments, carotenoids, and thylakoid polypeptides. To fully understand these processes, a knowledge of which polypeptides are coded in plastid DNA is of importance; we presently focus on two likely candidates, cytochrome c-552 and coupling factor CF-1, which may be coded in pDNA. Parallel studies of the three-dimensional relations of developing plastid membranes permit a more complete picture of the developmental process to be made.